This invention relates to a control system for a burner adapted to directly detect oxygen concentration in a room in which a burner is placed, to thereby prevent abnormal combustion due to a decrease in oxygen, and more particularly to a control system for a burner for detecting oxygen concentration in the room using an oxygen sensor adapted to react with gaseous oxygen to generate its output depending upon the concentration of oxygen such as a gas sensor of the galvanic cell type, to thereby generate an abnormality detecting signal to actuate at least one of an alarm and a combustion reducing device when the oxygen concentration in the room is lowered to a level below a predetermined set value.
The use of a combustion appliance or burner such as an oil-fired space heater, a gas-fired space heater or the like for heating a tightly closed room causes a reduction in oxygen due to combustion, leading to incomplete combustion of fuel in the burner. In view of the above, various techniques have been conventionally developed to detect a reduction in oxygen. For example, U.S. Pat. No. 4,710,125 proposes a system for indirectly detecting lack of oxygen based on a variation in flame current utilizing characteristics of a burner to generate an abnormality detecting signal. Also, a system wherein an oxygen concentration cell using zirconium is used for detecting a difference in oxygen partial pressure between air in a room in which a burner is placed and combustion gas discharged from the burner to generate an abnormality detecting signal is proposed. The systems proposed each are adapted to indirectly detect oxygen concentration using combustion taking place in a burner. Unfortunately, each of the proposed systems requires a relatively high voltage and a large current for the detection, therefore, it is required to use a commercial power supply. Thus, the proposed systems are not suitable for use for a burner wherein a natural draft is utilized for combustion and a dry cell or battery is used for a power supply, such as an oil burner of the wick actuating type. In view of the above, the inventors considered a system using an oxygen sensor, such as a gas sensor of the galvanic cell type, which can be driven through a dry cell or battery and is adapted to directly detect oxygen irrespective of combustion to generate an abnormality detecting signal.
An oxygen sensor for directly detecting oxygen in an environment such as a gas sensor of the galvanic cell type is generally adapted to determine oxygen within a wide range extending between 0% and 100%. Optimum oxygen concentration in a room in which a burner is placed is about 21%, whereas, in an abnormal condition due to consumption of oxygen by combustion, oxygen concentration is decreased to a level as low as 18% or less. This is generally called a low oxygen concentration condition. Thus, there is a significant difference in oxygen concentration between the optimum condition and the abnormal condition, therefore, measuring of a difference in oxygen concentration by means of the oxygen sensor causes an error because the oxygen sensor is not suitable for measuring oxygen concentration within such a narrow range with accuracy. In general, the oxygen sensor generally produces an error as large as 2 to 3% when measuring oxygen of, for example, 18% in concentration; thus, when the error is produced on the positive side, a measured value obtained by the oxygen sensor is as if oxygen concentration is within the normal range.
The major reason that the oxygen sensor which is a wet type gas sensor such as a gas sensor of the galvanic cell type and adapted to measure gas concentration according to an electrochemical procedure using an electrolyte produces an error in measuring of oxygen concentration would be that an ambient temperature of the sensor or a temperature of an environment surrounding the sensor adversely affects the oxygen sensor to vary the output of the sensor. More particularly, the oxygen sensor is generally adapted to output a degree of the reaction between a work electrode and oxygen in an atmosphere in the form of a variation in voltage, current or internal impedance. Unfortunately, the ambient temperature causes the reaction to be varied, resulting in the output of the oxygen sensor being varied. As another gas sensor which is adapted to directly measure oxygen concentration is used a gas sensor of the semiconductor type, which is likewise apt to be affected by the ambient temperature, to thereby be unsuitable for controlling the burner.
Further, the conventional various gas sensors including the gas sensor of the galvanic cell type generally exhibit characteristics distinctly different from those at a temperature at which they are usually used, when the ambient temperature is lowered to a level as near as the freezing point. Thus, it is highly desirable to eliminate such a problem.